As is generally known, an emergency radio communication and determination system employed in the case of distress is expected to allow transmission of messages from any point on the earth's surface for sufficiently large distances and without undue delay. The accuracy of determining the position of the distressed unit should provide for immediate guidance of vessels and aircraft participating in search and rescue missions. The system must permit simultaneous reception and separate processing of distress signals transmitted by dozens of distressed units in the common frequency band.
The emergency stations should be simple in construction, inexpensive and suitable for mass production.
These requirements impose severe limitations on the size and weight of the emergency stations as well as on power consumption for operation thereof.
Known from long-term world-wide experience are emergency radio communication and determination systems comprising emergency stations carried by mobile objects and actuated in case of emergency, and receiving stations provided on all sea vessels, ashore, and on search-and-rescue aircraft. AM and/or FM signals at international distress frequencies of 500 kHz, 2182 kHz, 121.5 MHz and 156.8 MHz are used by emergency radio stations for message transmission purposes. The same signals serve for position location of a distressed unit by means of radio direction-finding.
The disadvantages of such systems are their small coverage (at most 200 to 300 km), low reliability of communication, and insufficient accuracy of position location. These shortcomings may lead to search and rescue operations which are frequently both untimely and inefficient. They are largely caused by physical properties of the frequency bands indicated above, congestion of these bands, impossibility of using reliable and efficient antennae, and limited power resources of emergency stations.
Also known in the prior art is the OPLE system (See "Aviation Week and Space Technology", 1971, Aug. 23, No. 8, pp. 28 to 31) which comprises emergency radio stations including a receiver sensing the signals of the OMEGA Radio Navigation system and a transmitter for retransmission of the signals sensed by this receiver, geostationary satellites for retransmission of these signals, and ground-based receiving stations employed for receiving the signals retransmitted by the satellites and for calculation thereby of the emergency station position.
The disadvantage of this system resides in the complexity of the receiving and transmitting equipment and in the high emergency radio station power required, since high orbiting satellites are employed in the system. Furthermore, distress alerting in the system involves a complicated construction of the emergency stations, an extra power consumption of the stations and additional frequency bands.
Also known in the art is the SAMSARS emergency communication system (see the article by A. Weinberg et al. "A Novel Concept for a Satellite-Based Maritime Search and Rescue System", IEEE Journal of Oceanic Engineering, vol. OE-2, No. 3, July 1977) comprising emergency stations (radio buoys) for transmission of short distress messages, geostationary satellites for retransmission of these messages, and receiving ground stations. The message transmission is accomplished using a wideband binary phase-shift keyed (PSK) signal with a power of about 10 W, the pattern of the signals being the same for all the emergency stations. The use of this signal enables satellites incorporated into other systems, such as the Maritime communication System MARISAT, to be employed for retransmission of the messages, without interference to these systems as a result of operation in the common frequency band. The SAMSARS system is also capable of locating positions of the emergency stations, provided at least three geostationary satellites lie within the radio visibility area of these stations. It is then necessary that the arrival time of the signals received from the emergency stations be recorded at each receiving station using a single time scale.
Since the emergency stations of the SAMSARS systems transmit short distress messages with a small probability of their mutual overlapping, this system can accommodate several mobile objects being simultaneously in distress. The SAMSARS system, however, suffers from serious disadvantages including high power consumption of the emergency stations due to a great satellite-to-earth distance, impossibility to cover polar regions, i.e. the latitudes above 70 deg., and a low accuracy of radio determination near the equator, i.e. at the latitudes below 15 deg., which is accounted for by geometrical factors inherent in geostationary satellite-based systems. Moreover, another disadvantage of the SAMSARS system is the difficulty of radio determination requiring multiple widely spaced satellites in the geostationary orbit.
Another existing system for position location of distressed mobile objects (see J. D. Lambert, A. E. Winter "A Search and Rescue Satellite System (SARSAT)", Experiment Communication Research Centre, Ottawa, Ontario, Canada, 1976) comprises low-power emergency radio stations provided on the mobile objects and transmitting, in case of trouble, continuous sinusoidal signals for a prolonged interval, a low orbiting satellite equipped with a retransmitter for relaying of the signals from emergency stations, a control station for tracking the satellite and calculation of its ephemeris, a receiving ground station for reception of the signals retransmitted by the satellite and calculation of the coordinates of the emergency stations, and a communication link connecting the receiving station with the control station. The receiving station has a phase-locked receiver, a highly stable reference-frequency oscillator, a signal parameter measuring device, and a computer. The device for measurement of the signal parameters is a counter designed for counting the number of beats per unit time, resulting from mixing the received signals applied to one of its inputs from the output of the receiver and the reference-frequency oscillator voltage applied to its other input. The signal parameter measuring device delivers Doppler shift counts of the signal from the emergency station. These counts are introduced into the computer. Based on these counts and the ephemeris of the satellite obtained from the control station, the computer calculates the coordinates of the emergency station using well-known algorithms.
A major disadvantage of this system consists in that the receiving station is not capable of processing signals transmitted simultaneously by several emergency stations in the common frequency band, on account of mutual interference of the emergency stations. On the other hand, the assignment of a reasonable quantity of frequency-division-multiplex (FDM) or time-division-multiplex (TDM) channels for emergency service in a system incorporating hundreds of thousands of emergency stations, out of which number several dozens of stations may transmit distress signals simultaneously, is not found possible, since it is not known beforehand which emergency stations will transmit the distress signals and when.
Another disadvantage of the foregoing system lies in the fact that an additional signal is needed for transmission of the mobile object identification code and the data about the nature of the trouble, and consequently extra power of the emergency station and an additional frequency band are necessary.